In rotary drilling, a rock bit is threaded onto a lower end of a drillstring. The drillstring is lowered and rotated, causing the bit to disintegrate geological formations. The bit cuts a borehole somewhat larger than the drillstring, so an annulus is created between the walls of the borehole and the drill string. Section after section of drill pipe, or other drillstring tool, is added to the drillstring as new depths are reached.
During drilling, a fluid, often called “mud,” is pumped downward through the drill pipe, through the drill bit, and up to the surface through the annulus, carrying cuttings from the borehole bottom to the surface.
It is often useful to detect borehole conditions, drill bit conditions, and drillstring conditions while drilling. However, much of the desired data is not easily collected or retrieved. An ideal method of data retrieval would not slow down or otherwise hinder ordinary drilling operations, or require excessive personnel or the special involvement of the drilling crew. In addition, data retrieved in near real time is generally of greater utility than data retrieved after a prolonged time delay.
Directional drilling is the process of using the drill bit to drill a borehole in a specific direction to achieve some drilling objective. Measurements concerning the drift angle, the azimuth, and tool face orientation all aid in directional drilling. A measurement while drilling system may replace single shot surveys and wire line steering tools, saving time and cutting drilling costs.
Measurement while drilling systems may also yield valuable information about the condition of the drill bit, helping determine when to replace a worn bit, thus avoiding the pulling of bits that are not near their end of life or drilling until a bit fails.
Other valuable information may be gathered by formation evaluation within a measurement while drilling system. Gamma ray logs, formation resistivity logs, and formation pressure measurements are helpful in determining the necessity of liners, reducing the risk of blowouts, allowing the safe use of lower mud weights for more rapid drilling, reducing the risks of lost circulation, and reducing the risks of differential sticking.
Existing measurement while drilling systems are said to improve drilling efficiency. However, problems still remain with the transmission of subsurface data from subsurface sensors to surface monitoring equipment, while drilling operations continue. A variety of data transmission systems have been proposed or attempted, but the search for new and improved systems for data transmission continues. Such attempts and proposals include the transmission of signals through cables in the drill string, or through cables suspended in the bore hole of the drill string; the transmission of signals by electromagnetic waves through the earth; the transmission of signals by acoustic or seismic waves through the drill pipe, the earth, or the mud stream; the transmission of signals by way of releasing chemical or radioactive tracers in the mud stream; the storing of signals in a downhole recorder, with periodic or continuous retrieval; and the transmission of data signals over pressure pulses in the mud stream.
Drilling fluid telemetry in the form of continuous wave and mud pulse telemetry presents a number of challenges. As examples, mud telemetry has a slow data transmission rate, high signal attenuation, difficulty in detecting signals over mud pump noise, maintenance requirements, and the inconvenience of interfacing and matching the data telemetry system with the choice of mud pump, and drill bit.
Electrical telemetry using electrical conductors in the transmission of subsurface data also presents an array of unique problems. One significant difficulty is making a reliable electrical connection at each pipe junction. Communication systems using direct electrical connection between drill pipes have been proposed. In addition, communication systems using inductive coupling and Hall Effect coupling at drill pipe joints have been proposed.
With the ever-increasing need for downhole drilling system dynamic data, a number of “subs” (i.e., a sub-assembly incorporated into the drill string above the drill bit and used to collect data relating to drilling and drillstring parameters) have been designed and installed in drillstrings. For data transmission systems to operate to full advantage, it is desirable that drill string components, such as drill bits and sensor subassemblies, be produced to cooperate therewith. Drillstring components so configured could provide significant amounts of useful data. Unfortunately, such conventional subs are expensive and are configured as dedicated downhole components that must be placed in the drillstring instead of, or in addition to, a simple drill pipe or drill collar.
There is a need for new methods and apparatuses for distributing data processing modules along a drillstring and providing communication between these data processing modules and a remote computer. In addition, there is a need for methods and apparatuses for analyzing dynamic movements of the drillstring.